Restorative dentistry has made significant advances in the use of dental implants to support dental restorations intended to permanently replace natural teeth. Single-tooth restorations are possible using dental implants. Dental implants provide an alternative to removable dentures and eliminate the problems associated therewith such as poor fit, unsightliness, etc.
A dental implant restoration utilizes a jawbone mounted fixture adapted to be directly implanted in the alveolar ridge crest of a patient's edentulous jawbone after the gum tissue has been displaced. A coronal end of the fixture has at its gingival aspect a transverse surface that is flush or nearly flush with the alveolar ridge crest of the jawbone after the fixture has been implanted in it. The fixture is typically made from a biocompatible implant material such as titanium or a titanium alloy.
As described, for example, in U.S. Pat. No. 4,988,298 to Lazzara et al. and incorporated by reference herein, the dental implant restoration includes an abutment base used to accurately fix a permanent dental restoration (prosthesis) on the implant fixture. In this regard, the abutment base comprises a precision formed component adapted for securely carrying the prosthesis, while defining a precision formed abutment surface for seating against the gingival aspect of the fixture to orient the prosthesis in a predetermined and accurate manner within the patient's mouth. The prosthesis includes a supragingival substructure having a gingival end shaped for mating with the abutment base, in combination with an anatomical overlay which envelops the abutment base at its gingival end. Porcelain is most commonly used as the overlay material for aesthetic reasons.
In accordance with conventional procedures, the abutment base is first cast and then precision machined to include the abutment surface for precision fit with a gingival aspect transverse surface of the implanted fixture. The substructure of the prosthesis is then cast onto the abutment base, preferably at brazing temperatures, and the porcelain overlay is then formed, as by baking. The thus assembled dental restoration, with the abutment base fixed at the gingival end of the prosthesis substructure and overlay, is fastened to the implanted fixture with a screw or bolt. The screw is normally mounted within a central vertical bore formed in the prosthesis and is fastened into a threaded socket in the implanted fixture to clamp the abutment base tightly against the fixture. After screw installation, the central bore can be filled with a suitable dental amalgam which can be drilled out later if and when access to the screw is required to remove the restoration from the implanted fixture.
In the past, the abutment base for a jawbone anchored restoration has been constructed from a dental alloy such as a gold-based alloy of the general type described in the literature and intended for use in making crowns, etc. Such alloys, however, have not optimally met the unique requirements for an abutment base which desirably possesses a number of specific physical properties. That is, customary gold dental alloys though somewhat malleable even when hardened in order to accommodate chewing forces, are still harder (typically Vickers 300) than the abutment base alloy of the present invention in order to have good wear resistance. By contrast, the abutment base alloy desirably possesses sufficient ductility or malleability and tensile strength to accommodate the high stress and torque encountered at the connection site between a jawbone implant fixture and the dental prosthesis and prevent catastrophic brittle failures. Moreover, the abutment base alloy desirably has a high degree of machineability so that angular deviations in implant fixture position can be precision adjusted by shaping of the abutment base.
It has been known that the strength (tensile strength) of dental alloys can be increased with a high silver or silver-zinc content, but such approaches are not compatible with porcelain overlays due to discoloration (greening) thereof. Since the overlay in a jawbone anchored restoration is typically made of porcelain, and directly contacts the abutment base, a rigidified abutment base alloy with a high silver content or containing zinc would be aesthetically unacceptable. See, for example, Kropp, U.S. Pat. No. 4,007,040. Not only must the dental alloy be compatible with porcelain, it must be compatible with the dental implant fixture material, typically titanium or a titanium alloy.
It has also been known that strength can be attained in a dental alloy by additions of metals of the platinum group, such as platinum or palladium. A small amount of iridium in the alloys is also known to produce a fine grain structure. In recent years, because of the high price of gold and platinum, alloys have been employed in which part of the gold and platinum content is replaced by palladium. However, the gold and platinum content has remained relatively high in order not to sacrifice casting properties.
Accordingly, there exists a need for an improved dental alloy with a novel combination of physical properties which makes it useful in the casting of abutment bases. Specifically, there is a need for an abutment base alloy which is easily precision machined to adjust for deviations in positioning of a jawbone implanted fixture, which has high structural strength and ductility (toughness) to withstand the extreme stresses of an abutment base, which is compatible with the prosthesis overlay and fixture material, and which is relatively economical in comparison with known dental alloys. The present invention fulfills these needs and provides further related advantages.